Electrical distribution systems distribute electrical power from an electrical power transmission system to electrical power consumers. To protect and isolate electrical loads from abnormal operating conditions and allow electricians and engineers to safely work on and maintain an electrical distribution system, circuit breakers are deployed at various stages in the distribution system. For example, circuit breakers comprise part of the switchgear that is installed within power distribution stations and substations and are installed in panelboards at or near service drops of commercial buildings and residences.
A principal function of a circuit breaker is to protect its load and the electrical conductors in the load circuit from overcurrent conditions. In general, there are two types of overcurrent conditions: an “overload” and a “fault.” The National Electrical Code (NEC) defines an “overload” as: “operation of equipment in excess of normal, full-load rating, or a conductor in excess of rated ampacity that when it persists for a sufficient length of time, would cause damage or dangerous overheating.” A “fault” is defined as “an electrical connection, which is made unintentionally, resulting in an excessive amount of overcurrent.” Faults typically produce much higher currents than do overloads, depending on the fault impedance. A fault with no impedance is referred to as a “short circuit” or a “bolted fault.”
FIG. 1 is a simplified one-line drawing of a typical electrical distribution system 100, illustrating how conventional circuit breakers are deployed in the distribution system. Alternating current (AC) power supplied from the secondary winding of a step-down transformer 102 is connected to a first set of circuit breakers within a main distribution panel (MDP) 104. The first set of circuit breakers in the MDP 104 includes a main circuit breaker, which provides short-circuit and overload protection to all downstream loads in the system. The remaining circuit breakers in the MDP 104 serve to provide fault and overload protection to loads that are either directly connected to the MDP 104, such as motor load 106, or to one or more sub-panelboards 108, which include “downstream” circuit breakers (and possibly other sub-panelboards) that provide fault and overload protection to additional loads, such as motor load 110 and light load 112.
Conventional circuit breakers have been in widespread use for many years. However, there are various challenges and drawbacks relating to their use. One problem relates to the precision, both in terms of time and current, at which they are capable of responding to faults and other overcurrent conditions and the uncertainty that results due to their lack of precision. Conventional circuit breakers are electromechanical in nature and typically use some sort of spring mechanism to control whether line current is allowed to flow into their load circuits. Unfortunately, due to limitations on the magnetics and mechanical design involved, the time it takes, and the current level at which, a conventional circuit breaker trips in response to a fault can vary, even for a circuit breaker that is selected from a group of breakers having the same type and rating, and even among several circuit breakers of the same type and rating provided by the same manufacturer. The time-current precision of a conventional circuit breaker also tends to degrade and deviate over time, due to aging of its electromechanical components. Because of this variability, circuit breaker manufactures will often provide time-current characteristic data for each type and rating of circuit breaker that they manufacture. The time-current characteristic data of the circuit breaker is typically displayed in a two-dimensional logarithmic plot, such as illustrated in FIG. 2, with current on the horizontal axis, time on the vertical axis, and “tripped” and “not tripped” regions separated by an uncertainty band within which the trip status of the circuit breaker is uncertain.
In an effort to address the time-current uncertainties of conventional circuit breakers, electricians and engineers will often perform what is known as a “selective coordination study” when designing an electrical distribution system. The selective coordination study is usually performed prior to the electrical distribution system being constructed. The goal of the selective coordination study is to select and map circuit breakers in the distribution system design so that only the closest circuit breaker upstream from a fault or overload condition will trip in response to a fault or overload condition. A successful selective coordination study will help to ensure that only those sections of the electrical distribution system that are downstream from the source of the fault or overload condition are isolated and de-energized, allowing the remaining upstream sections of the distribution system to continue operating, despite the fault or overload condition.
A selective coordination study is performed taking into consideration the time-current characteristic data provided by the circuit breaker manufacturers. During the study, circuit breakers of different types and amperage ratings are selected and mapped into the design with the goal of preventing the uncertainty bands of the various circuit breakers from overlapping. Overlapping bands is undesirable since it provides an indication that one or more upstream circuit breakers may unwantedly or prematurely trip in response to a fault or overload condition, instead of a downstream breaker that is closer to the source of the fault or overload condition and which could otherwise fully isolate the fault or overload condition on its own.
There are software tools available in the prior art that display the uncertainty bands of the various mapped circuit breakers and which can assist electricians and engineers in performing selective coordination studies. Unfortunately, due to the uncertainty bands present in the time-current characteristics of the various mapped circuit breakers, the electrician or engineer will often determine that it is not possible to prevent one or more of the uncertainty bands from overlapping, as illustrated in FIG. 3. In order to address this problem, the circuit breakers must be rearranged and/or replaced with circuit breakers of different types and/or ratings.
Not only are selective coordination studies cumbersome to perform and time-consuming, they are also prone to error, particularly since human interpretation is involved. For example, when electrical generators and induction motors are part of the system design, assumptions must be made as to how current from such loads might possibly be injected into a fault when a fault occurs. Those assumptions are not always accurate, and the errors that follow, along with other errors that can take place in the selective coordination study, can be unwittingly translated into the actual construction of the electrical distribution system. Moreover, once a selective coordination study has been completed and the study is implemented in hardware, in practice, little adjustment can be made, except for replacing circuit breakers with other types of circuit breakers. Some conventional circuit breakers include mechanical adjustments, which allow the time-current characteristics of the circuit breakers to be manually adjusted once they have been installed. However, those adjustments are often inadequate at preventing the time-current uncertainty bands of the various circuit breakers from overlapping and upstream breakers end up tripping prematurely or unnecessarily, causing a larger portion of the distribution system to be de-energized than is necessary.